String Conversion

Converting between strings and numbers requires deliberate parsing or formatting. The strconv package provides the standard functions for these tasks.

Use strconv.Atoi to parse a decimal integer from text. It returns both an integer and an error; check that error before using the result. Use strconv.Itoa to format an integer into decimal text.

For other bases and numeric types, ParseInt, ParseFloat, FormatInt and similar helpers offer more control. Converting an integer directly with string(n) produces the UTF-8 representation of a Unicode code point, not its decimal digits.

Follow this diagram before executing the example. Check where a value or decision changes what runs next.

This example is a complete program. Save it as main.go or paste it into the Go Playground.

Go code

package main

import (
	"fmt"
	"strconv"
)

func main() {
	number, err := strconv.Atoi("42")
	if err != nil {
		fmt.Println("Invalid number")
		return
	}
	fmt.Println("Next:", number+1)
	text := strconv.Itoa(123)
	fmt.Println("Text:", text+"4")
	_, bad := strconv.Atoi("four")
	fmt.Println("Invalid input:", bad != nil)
}

Output

Next: 43
Text: 1234
Invalid input: true

Atoi parses "42" into integer 42, so addition produces 43. Itoa converts 123 to the string "123"; concatenating the character 4 produces "1234". Parsing "four" fails, so the final boolean is true.

• Do not use string(42) when you mean the digits "42"; use strconv.Itoa instead.
• Always check a parse error before trusting the resulting numeric value.

Parse the text "125", multiply its numeric value by two, then format the answer back into a string.

Tip: Edit the example above, predict the output, and run the modified code to test your reasoning.

You have practiced String Conversion in a small, self-contained program. The important habit is to predict what runs, inspect the actual output and distinguish the language rule from the particular example.

Before continuing, make sure you can explain the program to someone who has not seen it, including the line that changes its result.